An hour before an exercise session, the researchers gave study participants watermelon juice, watermelon juice with added L-citrulline, or a juice without L-citrulline. In both watermelon juice tests, the participants had less muscle soreness 24 hours after exercising.

There was no difference between plain watermelon juice and watermelon juice with added L-citrulline. That means the L-citrulline naturally present in the watermelon seemed to work better than the added amino acid.

They were also able to determine how much of the amino acid would have made it into the athletes’ blood stream using a test on cells in the lab and found that the compound seems to be more readily available to the body in its natural form, which could explain why the spiked watermelon juice didn’t work any better.

Previous studies have shown that watermelon juice also contains antioxidants, and seems to increase muscle protein and enhance athletic performance, the researchers note.